The Ohio State University requests a 5-year grant under the NICHD R24 Population Research Infrastructure Program to enable the Initiative in Population Research [IPR] to continue its highly successful support of population research. Since receiving an R21 award in July 2004, the IPR has grown substantially in the number of faculty affiliates, their disciplinary diversity, and their research productivity, making IPR a far stronger research center than 5 years ago. The growth of IPR has been accelerated by significant university investment in population research totaling nearly $5 million in faculty hires, renovation of new space for IPR, and seed grant support. Under the R24 award, IPR researchers will conduct a program of research in 7 thematic areas: 1) Union formation and dissolution; 2) Fertility; 3) Adolescent transitions; 4) Child health and development; 5) Adult health disparities; 6) Immigration/migration; and 7) Residential segregation. In each of these areas, IPR researchers are productive and innovative, and their work is supported by a substantial portfolio of research awards from NICHD, other NIH institutes, NSF, foundations, the state of Ohio, and other sources of funding. Many of the research projects are interdisciplinary collaborations among IPR researchers spread among 6 colleges and 14 departments at OSU, an outcome that IPR has deliberately fostered through a program of seed grants, working groups, and seminars. The proposed R24 infrastructure program will consist of three research support cores -1) Administration, 2) Statistics and Survey Methods, and 3) Geographic Analysis - and a 4) Developmental Core. Each core is carefully designed to provide the essential support and services that IPR researchers require to advance their programs of population research and that cannot be provided by their home departments. Key features of this infrastructure are a lean Administrative Core, a robust Development Core, and 2 technical cores that blend highly progressive technical services with strategic didactic activities. Emerging collaborations between behavioral and health scientists has been a notable development under the R21 award, and the program of activities under the R24 is designed to reinforce this trans-disciplinary mode of population research at Ohio State.